The distance between optical terminals of an optical fibre transmission system is limited by the optical power that can be launched into an optical fibre by optical transmitters in the optical terminals, loss and dispersion of the optical fibres interconnecting the optical terminals and the sensitivity of the optical receivers of the optical terminals. Where the distance between the desired end point of an optical fibre transmission system exceeds the maximum distance between optical terminals, optical amplifiers have been provided. Each optical amplifier comprises an input connector, for receiving the optical signal, connected to an Erbium doped fibre amplifier which is in turn connected to an optical connector for coupling the amplified optical signal into the fibre connected between the optical amplifier and the next optical amplifier or a terminal of the signal.
Clearly a low input power level will result in a short distance between optical amplifiers. As each of these amplifiers are expensive increasing the distance between repeaters is clearly desirable. However, the input of a signal with too high an optical power may result in the signal causing non-linear effects in the fibre, such as Self-Phase-Modulation, that can seriously degrade the signal. Such non-linear effects are especially severe at bit rates at or above 10 Gb/s and the onset of non-linear degradations can be quite sharp.
Therefore, the determination of a suitable input power which is high enough to maximise the distance between optical amplifiers and low enough to prevent the onset of non-linear effects is essential in the production and operation of a successful optical transmission system.
From a theoretical viewpoint this concept does not cause any difficulty. However, when implementing such an optical transmission system it becomes clear that slight variations in the power of an optical system can be caused by a variety of phenomenon and can result in an optical signal moving away from its optimum power level towards too high or too low a power. For example, sinusoidal variations may move the signal to too high a power level at the positive peak or too low a level at the negative peak. Power margins must be allocated in the design of the optical system so that during worse case transients and variations, when combined with other worse case conditions, the data remains error free. Allowing this margin reduces the available performance of a system and thus reduces the maximum allowed amplifier spacings.
Also, even within an appropriate power range, power transients can cause bit errors. For example, if a transient is faster than the automatic gain control in a receiver then the receiver electronics could be momentarily overloaded. These distortions can cause errors. During a transient the electrical signal, or eye, at the decision comparator will be larger or smaller than anticipated. This places the decision threshold at the wrong location in the eye which causes errors. Amplitude transients can cause phase transients in the clock recovery circuits that become jitter issues or even bit errors, as will be discussed in more detail below. Erbium doped fibre amplifiers can cause amplitude transients when employing several wavelengths at once. Consider the simple example of two wavelengths, if one wavelength is removed while the pump power remains constant, then the output power at the other wavelength will increase by 3 dB. The speed of the transient is determined by the pump power and by the response to the Erbium doped fibre and is measured in microseconds. Addition of a second wavelength causes a similar 3 dB drop in the output power of the first wavelength present. Also, Erbium doped fibre amplifiers can introduce transients due to mode-hopping in the pump laser of the amplifier. In addition amplifiers and other optical elements have a polarisation dependent gain or loss and changes in polarisation thereby cause changes in power level. Even vibrations and mechanical movement of optical fibres, connectors and other components of the optical path can alter the loss and so cause variations in the optical power.
An optically amplified link, can be several hundred kilometers long with many optical components such as optical cross-connects, optical amplifiers, couplers, cables, connectors, insulators, WDM filters and dispersion compensation components. When bit errors occur on such a link, it is not presently possible to determine the location or cause of the fault down to a single field replaceable unit. Such transients can cause as little as a 1% fluctuation in power and can arise with a frequency in the range of 0.1 to 1 MHz. Known optical transmission systems utilise so-called loss of signal (LOS) alarms such as the Bellcore TR-253 LOS alarm. With such an alarm the input optical power level is measured and compared to a threshold. One or two dB of hysterises is commonly used in the LOS alarm so as to prevent toggling alarms. A slightly more subtle detector has been implemented where the alarm is raised if the optical power level changes by more than 2 dB within fifteen minutes at the optical receiver, which terminates the optical link. Such an alarm gives an idea of the static power changes i.e. power changes over a long period of time of the order of minutes, but gives only a general indication that a change in power level has occurred somewhere in the optical system. However, this does not isolate where the problem has arisen nor does this give any information on variations and transients that are less than the 2 dB threshold nor transients which are of a much shorter duration.
A total absence of optical power, i.e. the signal is all zeros, for greater than 10 microseconds is also generally detected as a loss of signal alarm.
Another form of detector utilised to determine loss in an optical transmission system is an optical time domain reflectometer (OTDR) which is used to measure the loss in an optical link. However, optical isolators and variations in the level of scattering from optical fibres limit the accuracy of this measurement on complex links. If an OTDR is used while a system is in operation then it must be used at a wavelength that will not interfere with the operation of the system. This means that the loss measured is not the loss at the operating wavelength, which is the important loss, but is in fact the loss at another remote wavelength. An OTDR is useful in measuring substantial failures or changes in the loss of a moderate length of passive optical fibre, but is not generally useful for detecting amplitude transients in a complex optical path.
Thus the prior art consists of circuits that check for large variations in power over relatively long periods or the total absence of power over a shorter period.
However, there are no alarms or monitoring features where the detailed characteristics of power level transients are examined nor where the probable phenomenon causing the transients have been identified nor where the source of a transient is isolated to a field replaceable unit such as an optical cross-connect or an optical amplifier. In fact, there has been no prior art to date which even teaches the desirability of measuring so-called short duration or low level transients which are monitored and determined by the method and apparatus in accordance with the present invention.
U.S. Pat. No. 5,513,029 discloses a method and apparatus for monitoring the performance of optical transmission systems in which an optical signal is modulated with a low frequency dither signal to provide a modulated optical signal having a known modulation depth. This document discloses a method and apparatus therein which can detect power changes of the order of 1 dB or more and does not disclose a system which can monitor transients of a much lower level i.e. in the order of a 0.1% to 5% fluctuation and preferably in the order of 1% fluctuation. However, this document does disclose the concept of monitoring a transmission system periodically throughout the system and comparing the information obtained from these monitoring points.
In all optical systems utilising optical regenerators in place of traditional electrical regenerators, such as a SONET regenerator, the new devices do not acquire frame alignment nor measure bit error rate. These devices can suffer a problem when frame alignment and bit error rate measurement is not undertaken until the terminal at the end of an optical path. Therefore, when a phase transient is created by an optical element such as an optical regenerator, this results in degradation of the optical signal, and problems including loss of frame can ensue. At present the phenomenon causing the phase transient and the optical element in which it occurs can not be readily determined.
The present means of and apparatus for detecting transients also address this problem of phase transients.